Studies for applying a multiple-input multiple-output MIMO technique using multiple transmission/reception antennas to a relay technique have been actively progressed. For an MIMO wireless system, studies about spectrum efficiency and enhancement of communication safety have been intensively performed. Recently, a relay system is regarded as an effective technique for wireless fading lightening and enhancement of link performance. Accordingly, an MIMO relay system has been actively studied.
“MMSE based MIMO Cooperative Relaying Systems” Closed-form Designs and Outage Behavior” published in IEEE Journal on Selected Areas in Communications, vol. 30, pp. 1390-1401 (C. Song, K.-J. Lee, and I. Lee, September, 2012) was introduced as a relay transmission technique for minimizing mean squared error MSE of the conventional relay system. This considers total maximum transmission power constraint usable at a relay in a relay environment considering a direct link, particularly constrains summation of diagonal elements of a transmission covariance vector of the relay. However, this technique is not proper to apply a real system because it does not consider power constraint of respective antennas.